frequency division multiple accessfrequency division multiplexing
frequency division multiplexing
(FDM) The simultaneous transmission of multiple separate signals through a shared medium (such as a wire, optical fibre, or light beam) by modulating, at the transmitter, the separate signals into separable frequency bands, and adding those results linearly either before transmission or within the medium. While thus combined, all the signals may be amplified, conducted, translated in frequency and routed toward a destination as a single signal, resulting in economies which are the motivation for multiplexing. Apparatus at the receiver separates the multiplexed signals by means of frequency passing or rejecting filters, and demodulates the results individually, each in the manner appropriate for the modulation scheme used for that band or group.Bands are joined to form groups, and groups may then be joined into larger groups; this process may be considered recursively, but such technique is common only in large and sophisticated systems and is not a necessary part of FDM. Neither the transmitters nor the receivers need be close to each other; ordinary radio, television, and cable service are examples of FDM. It was once the mainstay of the long distance telephone system. The more recently developed time division multiplexing in its several forms lends itself to the handling of digital data, but the low cost and high quality of available FDM equipment, especially that intended for television signals, make it a reasonable choice for many purposes. Compare wavelength division multiplexing, time division multiplexing, code division multiplexing.
Last updated: 2001-06-28
Frequency-Hopping Spread Spectrum
(FH, FHSS) A variation of spread spectrum communications in which a sequence of pseudo random numbers control a frequency synthesizer, generating different carrier frequencies that "hop around" in the desired frequency range. The receiver tunes to the same sequence of carrier frequencies in synchronisation with the transmitter.Frequency hopping spread spectrum was invented by Hedy Lamarr ("the most beautiful girl in the world", Samson and Delilah etc.) and the composer George Antheil. They held a patent filed in 1942.
Last updated: 2009-07-01
(FM) A method of encoding data by varying the frequency of a constant amplitude carrier signal.Contrast Amplitude Modulation.
Last updated: 2001-04-02
Frequency Shift Keyingteleprinter messages by radio (RTTY) but can be used for most other types of radio and land-line digital telegraphy. More than two frequencies can be used to increase transmission rates.
Last updated: 1997-07-14